Structure and stability of B4, B4+ and B4- clusters

The structure and stability of B-4, B-4(+) and B-4(-) clusters have been investigated at the B3LYP/6-311+G*, MP2/6-311+G* and QCISD/6-311G* levels of theory. Four B-4, six B-4(+) and six B-4(-) isomers are identified. Of these 16 species, 11 have not been reported previously. The rhombic structures 1 and 1(+) are found to be the most stable on the neutral and cationic surfaces, respectively, in agreement with the results reported previously. The most stable B-4(-) isomer has an arrangement of atoms similar to the neutral 1 and cation 1(+). Natural bond orbital (NBO) analysis and MOs calculations suggest that there are multicentered sigma centripetal bonds in the structures 1, 1(+) and 1(-). The calculated nucleus-independent chemical shifts (NICS) of the structures 1, 1(+) and 1(-) are all negative values, which indicates their aromatic characters. The high degree of aromaticity and the multicentered sigma centripetal bond are responsible for the stability of the lowest-energy B-4, B-4(+) and B-4(-) isomers.